Clad sheet product

ABSTRACT

The invention relates to a clad sheet product, ideally suitable for automotive body sheet, including a core sheet and a cladding layer on one or both core sheet surfaces, the core sheet has an aluminium alloy of the AA6000-series alloys and at least one cladding consisting of an aluminium alloy of the AA6000-series alloys having a Cu-content of less than 0.25 wt. %.

FIELD OF THE INVENTION

The invention relates to an AA6000 or AA6xxx-series aluminium sheet capable of forming a flat hemming and the sheet can be applied as automotive body sheet.

BACKGROUND TO THE INVENTION

Generally, outer body panels of a vehicle require excellent physical properties in formability, dent-resistance, corrosion-resistance and surface quality. However, the conventional AA5000-series alloy sheets have not been favoured because they have low mechanical strength even after press forming and may also exhibit poor surface quality. Therefore, the use of AA6000-series sheet alloys have been increasingly used. The AA6000-series alloys provide excellent bake hardenability after painting and high mechanical strength as a result, thus making it possible to manufacture a more thin-gauged and more light-weight sheets in combination with a class A surface finish.

The vehicle body parts, such as the bonnet and the like, in general, are manufactured by the mechanical assembly between inner parts and outer parts. For example, the proper length of a flange is prepared and formed at the end of an outer panel of a vehicle. An inner panel is fixed onto the inside of the outer panel, and the flange of the outer panel is bent and folded to produce a mechanical binding. The whole process described is called “hemming”.

In the hemming process, it is preferred to perform a flat hemming (180° process), which has very strict processing conditions and a relatively low ratio (r/t) between the bending centre radius (r) and the thickness of a sheet (t). However, the bending property of AA6000-series sheet products is inferior to that of AA5000-series alloys. Thus performing a flat hemming at parts where press induced property is relatively high (i.e., a part with much transformation or deformation) results in a higher defect rate.

U.S. Pat. No. 5,266,130 discloses a process for manufacturing aluminium alloy panels for the automotive industry. The alloy includes as essential components broad ranges of Si and Mg and may also include Mn, Fe, Cu, Ti, etc.

U.S. Pat. No. 5,616,189 discloses a process for producing aluminium sheet for the automotive industry. The aluminium sheet produced from the alloys was subjected to a 5 hour pre-age treatment at 85° C. The aluminium sheet used in this patent was a continuous cast sheet and it is reported that sheet products produced by this route have been found to exhibit poor bendability.

U.S. Pat. No. 6,780,259 discloses a process for producing an aluminium alloy of the AA6xxx-series with carefully selected Mg, Si and Mn-contents and having a specific pre-age treatment resulting in improved bendability, including hemming, for use in forming panels for automobiles.

International application WO-98/24571 discloses a multilayer metal composite product obtained by compound strand casting. The product comprises a core, preferably an aluminium alloy, on at least one side of the core an interlayer bonded to the core and a cladding bonded to the interlayer. Depending on the application of the product, the cladding forming the outersurface of the composite product can be a aluminium brazing alloy for use in brazing sheet, an AA1xxx-series alloy to obtain a mirror like surface finish, or an zinc containing aluminium alloy or zinc or a zinc alloy to improve corrosion resistance.

In spite of these disclosures, there is a great need for selection of aluminium alloy sheets and methods for producing vehicle parts or members providing good strength and levels of formability which permit ease of forming into intricate parts without cracking.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an AA6000-series sheet product having consistent levels of hemming.

It is another object of the invention to provide an AA6000-series sheet product having consistent levels of hemming and sufficient strength for producing vehicle parts or members.

These and other objects and further advantages are met or exceeded by the present invention concerning a clad sheet product having a core sheet and a cladding layer on one or both core sheet surfaces, the core sheet consisting of an aluminium alloy of the AA6xxx-series alloys, and at least one cladding consisting of a different aluminium alloy of the AA6xxx-series alloys having a Cu-content of less than 0.25 wt. %.

As will be appreciated herein below, except as otherwise indicated, all aluminium alloy designations refer to the Aluminium Association designations in Aluminium Standards and Data and the Registration Records, as published by the Aluminium Association in 2006.

For this invention the term “sheet” or “sheet product” refers to a rolled product form over 0.15 mm through 2.5 mm in thickness with sheared, slit, or sawed edges.

For this invention the term “automotive body sheet” or “ABS” refers to aluminium alloy sheet for automotive body applications, in particular exterior panels, interior panels and structural parts.

For this invention the term “automotive” is meant to include automobile and other vehicular parts or members as described herein and other transport parts or members having similar construction.

For any description of alloy compositions or preferred alloy compositions, all references to percentages are by weight percent unless otherwise indicated.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides clad sheet product having a core sheet and a cladding layer on one or both core sheet surfaces, the core sheet consisting of an aluminium alloy of the AA6000-series alloys, and at least one cladding consisting of an aluminium alloy of the AA6000-series alloys having a Cu-content of less than 0.25 wt. %, and preferably less than 0.20 wt. %, and more preferably less than 0.10 wt. %

According to the present invention the properties of the core sheet are disconnected from the demands imposed on the surface of the clad sheet product. Thus, the core sheet alloy can be chosen such as to meet the mechanical demands as good as possible, whereas the cladding layer can be chosen such as to meet the demands on interaction with the environment and its forming characteristics in particular. The clad sheet product is capable of forming an improved flat hemming in comparison to the non-clad core sheet, and the clad sheet product can be applied as automotive body sheet.

By selecting an alloy for the cladding layer which is softer (or having a lower hardness) than the alloy of the core sheet the hemming behaviour is improved while maintaining the strength of the clad sheet product as a whole at a sufficiently high level as a resultant core sheet. The choice of the core material from the AA6000-series for the clad sheet product leads to excellent mechanical properties as these are substantially determined by the core sheet, and which are comparable with standard industrial sheet alloys used for this application. Tests conducted on specimens of the clad sheet product according to the present invention have shown further that there is no significant correlation between the thickness of the cladding and the mechanical properties, thus confirming what has been stated above with respect to disconnecting the bulk properties of the core material from the demands imposed on the cladding layer.

The Cu-content in the cladding layer is controlled to the indicated amounts in order to maintain a good corrosion performance and to control in particular the filiform corrosion resistance, which is an important property when the clad sheet product is used as automotive body sheet.

The use of an AA6000-series cladding results also in that the clad sheet product has an improved resistance against scratches during handling of the clad sheet product compared to an alternative AA1000- or AA3000-series cladding layer having a significantly lower hardness.

By using an AA6000-series both for the core and the cladding sheet, although different in composition due to the close similarity in alloy compositions the clad sheet product is very easy to recycle and allows some resilience in the scrap management when producing such a clad sheet product.

In an embodiment the core sheet is an AA6000-series alloy further comprising Cu up to 1.1%, and preferably up to 0.9%.

In a preferred embodiment the core sheet is a aluminium alloy selected from the group consisting of AA6016, AA6022, AA6056, AA6013, and AA6111-series alloy.

These aluminium alloys provide a good balance in strength after painting and baking, bendability and have sufficient strength to resist dents.

In a preferred embodiment the cladding layer on the clad sheet according to the invention comprises a low alloyed AA6000-series aluminium alloy, and preferably an AA6000-series alloy having a substantially balanced Mg₂Si composition to improve the bendability, the hemming performance and the crash performance of the clad sheet product according to this invention. A preferred aluminium alloy has a chemical composition comprising, in wt. %:

Si 0.40 to 0.9, preferably 0.5 to 0.9

Mg 0.40 to 0.8, preferably 0.4 to 0.7

Fe max. 0.35, and preferably <0.25

Cu <0.25, preferably <0.20, and more preferably <0.10

Mn <0.50

Cr <0.30

V <0.30

Zr <0.30

Zn <0.3, preferably <0.1

Ti <0.1,

others and inevitable impurities each <0.05, total 0.15,

balance aluminium.

A more preferred choice is the AA6005 alloy, and more in particular the AA6005A-series alloy, for both alloys with the proviso that the Cu-content is <0.25%, and preferably <0.20%. For the AA6005A alloy the sum of Mn+Cr is preferably less than 0.50%, and more preferably in a range of 0.1-0.50%.

The choice of this particular AA6005 or AA6005A-series aluminium alloy as cladding layer enables to obtain an excellent performance of the clad sheet product as regards to the environment. For example, its sensitivity for intergranular corrosion can be controlled within acceptable limits, and further its sensitivity for filiform corrosion, especially in combination with painted surfaces, can be reduced. The AA6005 and AA6005A-series alloys, in particular when having a balanced Mg₂Si composition, provide a significant better hemming performance than the AA6000-series core sheet.

Although the dimensions of the clad sheet product can be varied in many ways (mostly imposed by the specific use and concurrent demands), for the preferred use as automotive body sheet the core material has a thickness in the range of about 0.5 to 2 mm, preferably about 0.7 to 1.3 mm, and most preferably about 1 mm. The cladding layer or layers are much thinner than the core sheet, each clad layer constituting about 1 to 25% of the total clad sheet product thickness. A cladding layer more typically constitutes around about 2 to 14% of the total clad sheet thickness.

In an embodiment the cladding layer is applied on one core sheet surface only.

In another embodiment a cladding layer is applied on both core sheet surfaces, and preferably each cladding layer is made from the same AA6000-series alloy selected. As a result, the composite material exhibits excellent balanced properties, viz. strength and formability versus corrosion performance, dent resistance and hemming performance.

In an embodiment of the clad sheet product at least the AA6000-series core sheet, and preferably also the cladding layer, is brought to a T4 or T4P temper condition in which condition the clad sheet product is formed into a shaped panel or structural component. In this condition the clad sheet product provides the best balance in formability characteristics as evaluated by measuring strength, ductility, and the amount of deformation to cause failure. The “T4P” temper is condition wherein the sheet product is produced in a T4 process with pre-ageing. The pre-ageing treatment is the last step in the procedure prior to forming the clad sheet product into a shaped panel or structural component for a vehicle. A “T4P” indicated typically a process where the alloy sheet has been solution heat treated, pre-aged and naturally aged for at least several hours.

In a further aspect of the invention it relates to a formed automotive body panel, e.g. an interior panel or exterior panel or a formed automotive structural part or member, made from the clad sheet product according to this invention. Thus, clad aluminium sheet according to this invention provides a material having the strength and formability for use as vehicle or automotive sheet which can be formed into many different automotive structural members, such as dash panel, floor panel, door panel, reinforcements for panels, etc.

In a further aspect the invention relates to a method for producing a clad sheet product in which a cladding is applied to at least one side of a core material. In accordance with the present invention the core material comprises an aluminium alloy of the defined AA6000-series, and wherein the defined cladding sheet is attached to the core material by means of roll bonding to achieve the require metallurgical bonding between the core alloy and the cladding.

Such a roll bonding process is very economic and results in a very effective composite sheet material presenting the desired properties.

Of course, the roll bonding process may be accompanied by several additional processing steps such as for example annealing for obtaining the final properties of the automotive body sheet in the form of the clad sheet product.

When carrying out such a roll-bonding process for producing the clad products according to the present invention, it is preferred that both the core and cladding experience a thickness reduction during the role bonding.

For example the core material initially can be an about 400 mm thick block with at both sides a cladding initially about 24 mm thick. After roll bonding the final width of the core material was about 1 mm, whereas the final thickness of the cladding is, for example, about 60 μm.

It is noted that the initial dimensions and final dimensions of the clad sheet product will be determined both by the specifics of the roll-bonding process as well as the required properties of the final clad sheet product.

The roll bonding process can be carried out in different manners. For example, it is possible that the roll-bonding process includes both hot rolling and cold rolling.

Further, the roll-bonding process may be a one-step process or a multi-step process in which during successive rolling steps the material is gauged down. Separate rolling steps then may be separated by other processing steps, for example annealing steps, heating steps, cooling steps, etc.

In another embodiment according to this invention the defined cladding sheet is attached to the core material by means of a casting technique, for example as disclosed in EP-1638715, incorporated herein by reference.

As expressed above and set forth in the claims, the clad sheet product in accordance with the present invention is for use as automotive body sheet in vehicles.

The invention will now be illustrated with reference to non-limiting embodiments according to the invention.

EXAMPLE 1

On an industrial scale two different bare sheet products have been manufactured and processed to sheet products of 1 mm in a T4P temper. The two alloys were an AA6016 and an AA6111-series alloys, both of which are frequently used for automotive body sheet applications. Also on an industrial scale clad sheet products in the same temper using the same thermal history (for example homogenisation for 10 hours at 560° C.) have been manufactured but whereby the AA6016 and AA6111-series alloys were clad on both sides by means of regular roll bonding practice with an AA6005A-series alloy composition. Via hot-rolling the clad product has been rolled to an intermediate gauge of 7.5 mm and whereby the hot-mill exit temperature was about 300° C., and then cold rolled to a final gauge of 1 mm. The clad sheet had a total thickness of 1 mm and each clad layer had a thickness of 55 μm.

The exact alloy compositions of the AA6016 and AA6005A are listed in Table 1. All products have been solution heat treated at 560° C. and then quenched, and within 1 hour re-heated to about 80° C. followed by coil cooling to room temperature and then after 2 weeks at room temperature tested while being in the so-called T4P-temper for its strength, total elongation and hemming performance. The strength, intergranular corrosion resistance and dent-resistance have been measured after a simulated forming and paint-bake cycle whereby the product in the T4P-temper is further subjected to a 2% cold stretch following by a heat-treatment of 20 minutes at 185° C.

Both the bare sheet products and the clad sheet products have been tested in the T4P condition for their hemming performance via a flat hemming test (bending the samples 180° with a bending radius of 0.0 mm as included in ASTM norm E290-97A) and followed by a visually assessed. A score was given according to the following rating: rating “5” represents no visual defects, “4” mild surface roughening, “3” severe surface roughening, “2” small surface cracks, and “1” represents continuous surface cracks, and whereby a further sub-rating of for example 3¼, 3½ and 3¾ is used.

Furthermore the bare AA6016 sheet product and the AA6016 clad with AA6005A have been assessed for their mechanical properties according to ASTM norm EN10002 for tensile tests. The tensile properties have been determined in the referenced T4P-temper and also after a simulated forming and paint-bake cycle. The intergranular corrosion resistance (“IGC”) after the simulated forming and paint-bake cycle have been measured according to the European norm ASTM G110-92 and the result is expressed in penetration depth in μm. In addition the static dent-resistance after the simulated forming and paint-bake cycle has been measured whereby the product is mounted and subsequently loaded with a steel indentor having a radius of 63.5 mm with a speed of 2 mm/min, such that from the force-displacement curve the static dent-resistance F_(0.1 mm) is determined as the force (in N) required to make an indention of 0.1 mm deep. All test results are listed in Table 2, and whereby “n.t.” stands for “not-tested”.

TABLE 1 Alloy composition in wt. % of the AA6016 and AA6005A, balance aluminium and inevitable impurities. Alloy Si Fe Cu Mn Mg Cr Ti AA6016 1.0 0.23 0.15 0.07 0.60 0.03 0.02 AA6005A 0.6 0.20 0.09 0.12 0.55 0.01 0.01

TABLE 2 Test results of the bare alloy sheet and the alloy sheet clad with AA6005A. AA6016 with AA6111 with Bare Bare AA6005A AA6005A Property and condition AA6016 AA6111 clad clad Yield strength (MPa) in T4P 141 n.t. 137 n.t. Total elongation (%) in T4P 23.8 n.t. 24.7 n.t. Hemming in T4P 1¾ 1½ 3¼ 3¼ Yield strength after 258 n.t. 253 n.t. 2% + 185° C./20 min IGC (μm) after 118 n.t. 76 n.t. 2% + 185° C./20 min Dent-resistance F_(0.1 mm) (N) 260 n.t. 256 n.t. after 2% + 185° C./20 min

From the results of Table 2 it can be seen that by applying a thin clad layer of the AA6005A alloy the hemming performance of both the AA6016 and AA6111 are improved, while from the comparison of the bare AA6016 and the clad-AA6016 it can be seen that the intergranular corrosion resistance is significantly improved. These improvements are obtained while maintaining high strength levels and good formability as expressed in elongation, and without adversely affecting the dent resistance. It is believed that there is no difference in dent-resistance between bare AA6016 and clad AA6016 as the variation in within normal measurement inaccuracy.

In the present example the core alloys have been clad on both sides for practical reasons in the production thereof, but it will be immediately apparent to the skilled person that the same benefits can be obtained by using only a single clad layer.

This example illustrates the principle of the present invention that the hemming performance of an AA6000-series alloy suitable for automotive body sheet can be improved by providing it with a suitable cladding layer together with a significant improvement in the corrosion resistance, while still beneficing from the favourable characteristics of the core sheet such as its strength, the paint bake response and dent resistance.

The invention is not limited to the embodiments described before, which may be varied widely within the scope of the invention as defined by the appending claims. 

1. Automotive body sheet consisting of a clad sheet product consisting of a core sheet having a cladding layer on one core sheet surface only, the core sheet is of an aluminium alloy of the AA6000-series alloys and the cladding is of an aluminium alloy of the AA6000-series alloys having a Cu-content of less than 0.25 wt. %.
 2. Automotive body sheet according to claim 1, wherein the core sheet is an AA6000-series alloy further comprising Cu at most 1.1 wt. %.
 3. Automotive body sheet according to claim 1, wherein the core sheet is harder than the cladding layer.
 4. Automotive body sheet according to claim 1, wherein the core sheet comprises an aluminium alloy selected from the group consisting of AA6016, AA6022, AA6056, AA6013, and AA6111-series alloy.
 5. Automotive body sheet product according to claim 1, wherein the cladding layer has a chemical composition comprising, in wt. %: Si 0.40 to 0.9 Mg 0.40 to 0.8 Fe max. 0.35 Cu <0.25 Mn <0.50 Cr <0.30 V <0.30 Zr <0.30 Zn <0.3 Ti <0.1, others and inevitable impurities each <0.05, total 0.15, and balance aluminium.
 6. Automotive body sheet according to claim 5, wherein the Cu-content in the cladding layer is <0.20%.
 7. Automotive body sheet according to claim 5, wherein the Fe-content in the cladding layer is <0.25%.
 8. Automotive body sheet according to claim 5, wherein the Si-content in the cladding layer is in a range of 0.5 to 0.9%.
 9. Automotive body sheet according to claim 5, wherein the Mg-content in the cladding layer is in a range of 0.4 to 0.7%.
 10. Automotive body sheet according to claim 1, wherein the cladding layer is of aluminium alloy AA6005 or AA6005A, each with the proviso that the Cu-content is <0.25%.
 11. Automotive body sheet according to claim 1, wherein the core sheet has a thickness in the range of 0.5 to 2 mm. 12-13. (canceled)
 14. Automotive body sheet according to claim 1, wherein the cladding layer has a thickness in the range of 1 to 25% of the total thickness of the clad sheet product.
 15. Automotive body sheet according to claim 1, wherein at least the core sheet is in a T4 temper or T4P temper.
 16. Automotive body sheet according to claim 1 wherein each core sheet and cladding layer are made of an AA6000-series alloy of different composition.
 17. Formed automotive structural member made from the automotive body sheet according to claim
 1. 18. Formed automotive structural member according to claim 17, wherein said member is an exterior body panel.
 19. Automotive body sheet according to claim 1, having a Cu-content of less than 0.20 wt. %.
 20. Automotive body sheet according to claim 1, wherein the core sheet is an AA6000-series alloy further comprising Cu at most 0.9 wt. %.
 21. Automotive body sheet according to claim 5, wherein the Cu-content in the cladding layer is <0.10%.
 22. Automotive body sheet according to claim 1, wherein the cladding layer is of aluminium alloy AA6005 or AA6005A, each with the proviso that the Cu-content is <0.20%.
 23. Automotive body sheet according to claim 1, wherein the cladding layer has a thickness in the range of 2 to 12% of the total thickness of the clad sheet product. 